This invention relates to an ABS or ABS-type composition, particularly, it relates to an ABS or ABS-type composition having a trimodal rubber particle size distribution.
It is well-known in the art that impact resistant polymer compositions can be prepared by incorporating therein various elastomeric (rubber) materials.
One such impact resistant composition is a terpolymer of acrylonitrile/butadiene/styrene (ABS). ABS or ABS-type compositions normally comprise a combination of elastomer (e.g., polybutadiene) with a rigid interpolymer of monovinylidene aromatic monomer and an ethylenically unsaturated nitrile monomer. ABS or ABS-type compositions usually consist of a rigid matrix as a continuous phase having dispersed therein particles of the elastomer. The elastomeric particles usually have grafted thereto amounts of the rigid interpolymers or a similar inter- or homopolymer.
The physical properties of ABS or ABS-type interpolymers are greatly affected by the concentration, size, and structure of the elastomeric particles contained therein. Particles 0.5.mu. in diameter or larger contribute greatly to impact strength and reduce the gloss of the composition. Particles smaller than about 0.5.mu. in diameter result in glossier compositions, said glossy compositions having less impact strength than those compositions in which larger particles (0.5.mu. in diameter or greater) are used in similar amounts.
Similarly, the structure of the individual elastomeric particles affects the physical properties of the ABS or ABS-type interpolymers. There are, generally, two main structure types of elastomeric particles.
One of the two main types is a grafted rubber particle containing occlusions of matrix polymer therein. Rubber particles with this structure are usually formed and grafted in a mass, mass-type, mass-suspension-type, or mass-solution-type, polymerization process wherein phase inversion occurs. These processes normally involve dissolving a previously-produced rubber in an amount of polymerizable monomer or in a mixture or solution of polymerizable monomer(s) with optional diluents, which monomer(s) are then polymerized under agitation. These grafted, occlusion containing rubber particles prepared by a mass, mass-solution-type, mass-suspension-type, or mass-type process will hereinafter be referred to as "mass polymerized particles". The mass polymerized particles impart impact resistance to ABS or ABS-type compositions. Detrimentally, the mass polymerized particles lower the gloss of the interpolymer in which they are contained.
The other main rubber particle structure type is a generally solid rubber particle. Rubber particles with this structure are usually formed and grafted through an emulsion polymerization process. The emulsion polymerization process generally involves emulsion polymerization of a rubber in an aqueous latex, then adding polymerizable and graftable monomers to the rubber-containing latex and polymerizing said monomers to form the graft portion and all or part of the desired matrix polymer. These generally solid rubber particles will hereinafter be referred to as "emulsion polymerized particles." An interpolymer containing emulsion polymerized particles will generally exhibit less impact resistance than an interpolymer containing an equal amount of rubber in the form of mass polymerized particles.
It is desirable to produce an interpolymer possessing good impact resistance. In an attempt to produce such a composition much work has been done in the area of tailoring the size, structure, and concentration of the rubber particles. Representative Patents include U.S. Pat. Nos. 3,903,200; 4,012,462; 4,277,574; and 4,430,478.
U.S. Pat. No. 3,903,200 teaches a polymeric composition having dispersed therein rubber particles having a bimodal size distribution. One set of particles is produced in a mass-polymerization process and has an average particle size of 0.7 to 10 microns. The other set of particles is produced in an emulsion polymerization process and has an average particle size of 0.01 to 0.5 microns.
U.S. Pat. No. 4,012,462 teaches a method of producing a rubber modified polymeric composition wherein the rubber particles have a broad particle size distribution.
U.S. Pat. No. 4,277,574 teaches a method of producing a rubber modified polymeric composition. The rubber modifying particles are produced in a mass process and have a bimodal particle size distribution.
U.S. Pat. No. 4,430,478 teaches a polymeric composition having dispersed therein rubber particles having a trimodal particle size distribution. One set of particles is produced in a mass-polymerization process and has a diameter of from 0.5 to 5.0 microns. The second set of particles is produced in an emulsion polymerization process and has a diameter of from 0.05 to 0.18 microns. The third set of particles is produced in an emulsion polymerization process and has a diameter of from 0.25 to 0.6 microns.
The prior art recognized that the presence of a large mass polymerized particle improved the impact resistance of an ABS resin. The prior art measured impact resistance through a Notched Izod or Falling Dart impact test. These tests only measure impact resistance within a narrow range of impact rates.
Impact resistance for a polymeric composition can be determined by impelling a dart into a sample of the composition and measuring the energy needed to break the sample. It has been found that the amount of energy needed to break a sample of a given composition changes as the impact speed of the dart changes. It is therefore possible for a given composition to have better impact resistance when impacted by a dart travelling at a high rate of speed than when impacted by a dart travelling at a low rate of speed. The converse can also be true.
It would be desirable to produce an ABS or ABS-type composition having good impact resistance over a broad range of dart impact speeds. It is to this goal that the present invention is directed.